Image blocking is increasingly popular with Internet service providers and individuals. Blocking occurs when either the service provider or the e-mail client an individual uses stops images from displaying when the e-mail is being read. As you can imagine, this can have a critical effect on an e-mail campaign if the creative depends chiefly on images to convey its message.

Usually the image is replaced with a border and a small red "x" mark in the upper left corner. The newest versions of Outlook (Outlook 2003 and Outlook Web Access) and other e-mail clients (AOL versions 6 through 9, and Thunderbird) are installed with image blocking enabled by default.

Therefore, recipients who use these e-mail clients must turn the image blocking off so they can view images you send in an e-mail. Believing that users know they need to turn blocking off and can accomplish this task is usually a long stretch.

Additionally, e-mail service providers such as Yahoo are giving users the ability to block images when using their Web e-mail viewers. Again, some providers such as Google's Gmail service have image blocking turned on by default. Similarly, you are relying on the recipient to be able to turn image blocking off.

AOL goes one step further by blocking the entire e-mail and returning a non-delivery message to the sender. With AOL's method, the user must activate the filter - but after it is activated, the intended recipient does not even know he is missing e-mail.

Another issue with image blocking will cause the open rates, based on image access, to undercount true opens that may be occurring. Typically, e-mail opens are tracked by counting the accesses on an image or through the use of a Web beacon program. These images and beacons are placed in the HTML code within the e-mail creative and inside <img> tags. These are the same <img> tags that are blocked when recipients have image blocking enabled. Therefore, image blocking defeats e-mail open tracking.

Even when images are blocked, there are methods to track opens, though the solutions do not work across the board with all e-mail service providers and e-mail clients.

HTML allows the Web beacon program to be placed in other HTML tags such as the <link rel="stylesheet"> which has an src attribute that can be used to call the Web beacon program. (The src attribute is where the "source" file is specified in the HTML tag.) Using the <link rel> tag in this manner works with Outlook 2003 and Hotmail, but does not work with Gmail or Yahoo.

Because of the variation among ISPs, e-mail service providers and e-mail clients, diligence must be taken to continually monitor the e-mail marketing environment for tags that work and those that do not. Because images used to convey meaning might be blocked, attention should be given to other means within the ad copy to accomplish marketing goals.

Creative copy in an e-mail can be formatted with HTML code to convey messages through the use of font size, color and style. This type of HTML will display properly in all e-mail viewers even when image blocking is enabled.

By using bold fonts, color and great copy, e-mail creative still can move a recipient to action. When developing e-mail marketing creative, pay attention to how the e-mail displays with and without images.

Many e-mail marketers try to circumvent the image-blocking problem by asking the e-mail recipient to add the e-mail URL to the e-mail client address book and thereby join the user's e-mail client whitelist. This can be effective, but only under some circumstances can you persuade the recipient to do this, and only if the e-mail reaches the intended recipient in the first place.